FIG. 4 is a front view of a main part of a conventional printed board machining apparatus, FIGS. 5A and 5B are drawings showing a collet chuck section of the printed board machining apparatus, wherein FIG. 5A is a front section view thereof and FIG. 5B is a view taken in the direction of an arrow A in FIG. 5A. FIG. 6 is a front section view of a tool holding unit.
In FIG. 4, an X table 1 is movable in an X direction perpendicular to the face of the sheet. A tool holding unit 20 and a printed board 15 are placed on the X table 1. A Y table 2 is movable horizontally in a Y direction in the figure. The Y table 2 holds a Z table 3 so as to be movable vertically in the Z direction in the figure by a motor 4 having an encoder and a ball screw 5. The Z table 3 holds a spindle 6 that rotatably holds a tool (here an end mill) 8 through the intermediary of a collet chuck 7. An NC unit 9 controls rotation of the motor 4 to accurately position an edge 7a of the collet chuck 7.
As shown in FIGS. 5A and 5B, the collet chuck 7 has a hole 7b formed at the center thereof and slits 7s having a width of around 0.5 mm and provided at the edge portion thereof. The portion where the slits 7s are provided will be referred to as a claw portion 7d hereinafter (note that only the claw portion 7d is shown in FIG. 5A). The claw portion 7d is provided with a channel 7m in a middle part thereof. A pad 10 made of an elastic member is disposed in the channel 7m. A thickness of the pad 10 itself is thicker than a width of the slit 7s, so that the pad 10 urges the claw portion 7d in a direction of widening the slit 7s. 
An outer periphery of the claw portion 7d is formed so as to have a tapered plane 7f having the same angle as a tapered plane 6f of the spindle 6. When the tapered plane 7f is separated from the tapered plane 6f by moving the collet chuck 7 downward in the figure by means not shown, the claw portion 7d is opened due to the resilience of the collet chuck 7 itself and the pad 10, and a diameter of the hole 7b becomes larger than that of a shank 8a of the end mill 8. When the collet chuck 7 is moved upward in the figure by the means not shown, and when it is urged further upward in a state when the tapered plane 7f abuts against the tapered plane 6f, the claw portion 7d is urged in a direction of narrowing the slit 7s, thus holding the shank 8a rigidly with frictional force.
The Z table 3 supports a pressure foot 11 by means of a pair of cylinders 12 that urges the pressure foot 11 downward in the figure. A ringed brush 13 is disposed at a bottom face of the pressure foot 11. It is noted that the pressure foot 11 is located at its rising end in the figure.
As shown in FIG. 6, a base 21 of a tool holding unit 20 supports a holder 22 so as to be movable in the vertical direction. A spring 23 urges the holder 22 upward. The holder 22 is provided with, at the center part thereof, a hole 22a whose diameter is larger than an outer diameter of the shank 8a of the end mill 8, i.e., the tool, or larger than a blade portion 8t when an outer diameter of the blade portion 8t is larger than the outer diameter of the shank 8a. Still more, the holder 22 is configured so that its upper face 22u reaches a preset height from the surface of the X table 1.
A ring 30 is made of hard synthetic resin, for example, and has a hole 30a which is provided at the center thereof and whose diameter is slightly smaller than the outer diameter of the shank 8a. As a result, when the shank 8a is inserted into the hole 30a, the ring 30 is combined in a body with the end mill 8 due to its resilience. That is, the ring 30 is a tool positioning device of the end mill 8. The holder 22 supports the end mill (machining tool) 8 attached with the ring 30. It is noted that the ring 30 has an outer diameter of around 7 to 8 mm and a height h of around 4 to 8 mm, and the shank 8a has the outer diameter of around 3 mm.
Next, an operation of the conventional printed board machining apparatus will be explained.
At first, a procedure for holding the end mill 8 with the spindle 6 will be explained. It is noted that the holder 22 supports the end mill 8 in a state in which it is inserted into the ring 30. Still more, the ring 30 is fixed with the end mill 8 so that a distance from a bottom face 30d thereof to a point 8p of the end mill 8 becomes a distance k. Accordingly, because a height h of the ring 30 in the vertical direction is preset, a distance from an upper face 30u of the ring 30 to the point 8p of the end mill 8 is determined.
The X and Y tables 1 and 2 are moved so that the axial center of the collet chuck 7, i.e., the spindle 6, coincides with an axial line of the hole 22a. Then, the Z table 3 is moved downward while opening the collet chuck 7 to position the edge 7a of the collet chuck 7 at position N indicated by a two-dot-chain line in FIG. 6. When the collet chuck 7 is closed in this state, the end mill 8 is held in a state in which the edge 7a of the chuck 7 contacts with the upper face 30u of the ring 30.
Next, a procedure for machining a channel on the printed board 15 will be explained.
After positioning the axial center of the end mill 8 to the center of the channel to be machined, the Z table 3 is moved downward while supplying air to the center part of the spindle 6 so that the point 8p of the end mill 8 cuts to a predetermined depth of the printed board 15. Then, the X and Y tables 1 and 2 are moved in accordance to a machining program. The air supplied to the center of the spindle flows out of the slits 7s of the collet chuck 7 and prevents chips from entering the slits 7s. 
Because the use of the ring 30 enables one to accurately control the position of the point 8p of the end mill as described above, the cutting depth can be set at a required minimum value in machining the printed board, thus improving the machining efficiency. It also causes no insufficient cutting of the end mill 8. Still more, it becomes unnecessary to confirm the position of the point 8p during machining.
By the way, the life of the tool may be prolonged by suppressing an increase of temperature of the tool during machining. The prolongation of life of the tool not only improves the machining efficiency but also reduces running cost.
A linear and concave coolant supplying section in parallel with an axial line, from the rear end of the shank portion to a cutting face of the blade portion, to directly apply the coolant to the cutting face of the blade portion of the end mill is disclosed in Japanese Publication Patent No. 2894924. A plurality of independent cutting blades disposed in spiral in the axial direction on a rectangle whose width can be defined in the axial direction, so that an interval in the axial direction is wider than the cutting blade, is disclosed in Japanese Patent Laid-Open No. 2002-337016. A blow outlet of compressed air is disposed in the vicinity of the tool as disclosed in Japanese Patent Laid-Open No. 1997-117815.
However, because the channel is provided on the shank portion in both the conventional technologies disclosed in Japanese Publication Patent No. 2894924 and Japanese Patent Laid-Open No. 2002-337016, the unit cost of their tools becomes expensive as compared to a standard tool. Still more, because the structure of the tool disclosed in Japanese Patent Laid-Open No. 1997-117815 is complicated, its maintenance and inspection are cumbersome.
Accordingly, it is an object of the invention to provide a tool positioning device and a machining element which are capable of solving the above-mentioned problem, whose structures are simple, and which allow a running cost to be reduced.